Backlash compensation circuit for numerically controlled stepping motor driven machine

ABSTRACT

A circuit for use with a numerically controlled machine having lost motion or backlash between a moved part and a stepping motor which enables selection of (1) no backlash compensation, (2) backlash compensation by moving the part to each final position in the same direction or (3) backlash compensation by adding the exact amount of backlash each time the movement of the part is reversed. The circuit has a counter which in selection (2) adds and subtracts a movement greater than the actual backlash while in selection (3), the counter is set to provide the exact amount of backlash and also have its condition used to determine when backlash compensation is required.

STEPPING MOTOR DRIVEN MACHINE [75] Inventor: Albert C. Leenhouts,l-larwinton,

Conn.

[73] Assignee: The Superior Electric Company,

Bristol, Conn.

[22] Filed: Oct. 4, 1971 [21] Appl. No.: 186,015

[52] US. Cl. 318/630 [51] Int. Cl. G051) 11/01 [58] Field of Search318/630, 696

[56] y References Citedw UNITED STATES PATENTS 3,560,830 2/1971Steinberg 318/630 3,355,642 11/1967 Leenhouts 318/630 3,172,026 I 3/1965Schuman 318/630 X v United States Patent H 1 1 3,766,461 Leenhouts 1Oct. 16, 1973 1 BACKLASl-l COMPENSATION CIRCUIT 3,209,223 9/1965 Scheib,Jr. 318/630 x FOR NUMERICALLY CONTROLLED 2,913,648 11/1959 Brouwer318/630 X Primary Examiner-T. E. Lynch AttorneyArthur A. Johnson et a1.

[57] ABSTRACT A circuit for use with a numerically controlled machinehaving lost motion or backlash between a moved part and a stepping motorwhich enables selection of (1) no backlash compensation, (2) backlashcompensation by moving the part to each final position in the samedirection or (3) backlash compensation by adding the exact amount ofbacklash each time the movement of the part is reversed. The circuit hasa counter which in selection (2) adds and subtracts a movement greaterthan the actual backlash while in selection (3), the counter is set toprovide the exact amount of backlash and also have its condition used todetermine when backlash compensation is required.

4 Claims, 1 Drawing Figure 2/ 9 START m DOWN COUNTER Mow? ii 51?? SET TONUMBER CONTROL X OF PUL-EES 1 PES 1 RED LOST INPUT -=o 4 MOTION /6 //5 Nlo COUNT L MACHINE 7. PART DIRECTION +x lfifi (fig 46 i+ 5 BIT UP-DOWNCOUNTER 2 UP 5 42 -'nowu 26 Z 1 2? I (COUNT E RESET m o N9 0 Pmmmm 16 ms3.766461 PULSES m DOWN COUNTER MOTOR 551%? SET TO NU BE CONTROL 0FPULSES I mas: RED 95 LOST INPUT N=O 4 MOTION /5 N 9* 0 l6 L COUNT KMACHINE 6 PART DIRECTlON i'fifi +x Z5 fifia 5 BIT UP-DOWN COUNTER INVENTOR.

4 7156/ 2 C lenkou 1 6 WM m BACKLASH COMPENSATION CIRCUIT FORNUMERICALLY CONTROLLED STEPPING MOTOR DRIVEN MACHINE The presentinvention has particular utility when used with a numerically controlledmachine tool which has a part that is to be accurately moved by astepping motor. In many instances there is lost motion or backlashbetween the part which may be thebed of a milling machine and the motor.The motor must have a backlash compensating movement in addition tocommanded movements in order to assure that the part is moved exactlythe commanded distance for each movement.

One manner of accommodating backlash is disclosed in my U.S. Pat. No.3,355,642, assigned to the assignee of the present invention wherein thepart is always moved to its final position in one direction. For commandmovements in the one direction, no additional motor movements are madefor backlash compensation while for command movements in the otherdirection, a first movement in the other direction is made that isgreater than the existing backlash and then a compensating movement ofthe same extent is made in the one direction as the last movement afterthe commanded movement. In U.S. Pat. No. 3,560,830, there is disclosedanother form of backlash compensation in which the extent of thebacklash is first determined and with each command movement that is inthe opposite direction from the preceding movement, an additionalmovement in the commanded direction equal to the extent of the backlashis executed, prior to executing the command movement.

In both of the above-noted patents, as in the present application, thenumerical control system utilizes a stepping motor which moves the parta predetermined distance for each pulse received by the motor control.Accordingly, both systems utilize a settable counter for providing thepulses needed for the backlash compensating movements.

It is accordingly an object of the present invention to provide anumerical control system having a backlash circuit which enables a userto select to either exclude backlash compensation, or to effect backlashcompensation by moving the part in the same direction to its finalposition or to compensate for backlash by having the exact amount ofbacklash included in each movement reversal.

Another object of the present invention is to achieve the above objectswith a circuit that is relatively simple in construction, composed ofrelatively few parts and which may be easily incorporated into presentlyexisting numerically controlled systems.

A further object of the present invention is to provide backlashcompensation in a numerical control system by the use of a circuit whichincludes a counter that is both capable. of supplying additionalcommands for backlash compensation that is greater than the actualbacklash compensation or which may supply commands for the exact amountof compensation in addition to determining in the latter instance ifbacklash compensation is required.

The present invention may be advantageously utilized in a numericalcontrol system such as shown in U.S. Pat. No, 3,466,515 also assigned tothe assignee of the present invention, which has a stepping motorconnected to a machine part and causes the part to be moved in equallength increments or steps for each change of energization applied tothe motor. The system accepts as commands, the number of changes ofenergization or steps required for a movement either by manual settingsor by the use of coded records such as punched tape and translates itinto the number of changes of energization to the motor. After thenumber of changes have been supplied, the motor has produced therequired movement but the part to which it is connected may have moveddifferently by reason of backlash between the motor and the part.

To overcome the backlash error as it is the precise movement of the partthat is required, the present invention has a settable switch which inone position does not provide for backlash compensation while in anotherposition it achieves compensation as described in my above-noted U.S.Pat. No. 3,355,642. Additionally, in a third position the exact amountof backlash compensation is included with each change of direction ofthe part. More specifically the system includes a logic circuit and afive bit up down counter (having a maximum count of, for example, 32through a different numbered counter may be employed if desired). The

count of the counter is used to control the extent of the compensatingmovement and aso the counter count is sensed and used to determine ifthe reverse movement backlash compensation is required.

Other features and advantages will hereinafter appear.

I the present invention.

Referring to the drawing the backlash compensating circuit is generallyindicated by the reference numeral 10 and is shown interconnected withportions of a numerical control system 11 of the type which has astepping motor 12 that is connected to'precisely move a machine part 13.As indicated by the block 14 there is lost motion or backlash betweenthe movement of the stepping motor 12 and the machine part 13. Thenumerical control system includes a binary coded decimal down counter 15having an input 16 which sets the count of the down counter to thenumber of pulses desired for the stepping motor to move on a line 16aand also the direction of movement either +X or --X on a line 16b.

The down counter 15 is provided with two outputs indicative of itscondition, one-output being connected to the terminal A denoted N 0 andit supplies a logic 1 or high voltage level only when the count of thecounter 15 is zero. If the count is not zero, then there is a 1 or highvoltage on a lead A otherwise there is a logic 0 or low voltage level.In addition, at a terminal 17 denoted count, positive pulses arereceived by the counter 15 and for each pulse the counter will decreaseits numerical count by one.

The pulses in the system are derived from an oscillator 18 which has itsoperation initiated by a command on a start lead 19 to supply pulses ona lead 20 to a motor control 21 which translates .each pulsereceivedinto a change of energization of the stepping motor 12 andaccordingly an incremental movement thereof and of the part 13. Themotor control 21 has apair of gates 22 and 23 each having an inputconnected to the pulse lead 20 and indirectly to the direction command16b of the counter 15 such that when the input direction is, forexample, the gate 22 will permit the pulses to pass to the motor control21 as indicated by the symbol +X to cause the stepping motor 12 to movethe part in this direction while if the input direction command isindicating a movement in a reverse direction, the gate 23 permits thepulses to pass to the motor controller on the lead denoted -X to causethe motor to be energized to move in the other direction. The directioncommand consists of a l or high positive voltage for a +direction and alack of the voltage, i.e., logical for the direction and thisinformation controls which gate 22 and 23 each pulse will pass. I

It will also be understood that in normal operation each of the pulsesdelivered to the motor control 21 is also delivered to the countterminal 17. The down counter decreases its count by one for each pulsereceived until its count becomes zero at which time the oscillator l6isprevented from supplying further pulses to the motor control. Thenumerical control circuit has caused the stepping motor 12 to thus takethe number of steps to which the down counter was commanded by the input16a and in the direction indicated by the input command 16b.

The above-described circuit is basically disclosed in my US. Pat. No.3,355,642, it being understood that information to the down counter maybe supplied in any desired manner as by settable switches or codedrecords such aspunched tape, etc.

The backlash circuit includes a mode switch 24 having a first terminal25 which, if selected, prevents any backlash compensation; a terminal Gwhich, if selected, provides backlash compensation by moving the part toeach final commanded position in the same direction and a third positionwherein'there is a connection to a terminal H which adds the exactamount of lost motion 14 to each command movement whenever there is areversal of movement. There is additionally provided a binary codeddecimal five bit up-down counter 26 having a count terminal 27 on whichpulses are received with each pulse changing the count of the counter byone. A or logical l voltage at the terminal N causes the counters countto increase by one for each'pulse while a logic 1 voltage at theterminal C causes thecounte'r to decrease its count by one for eachpulse. A switch 28 is connected to the counter and when operated setsthe counter to a .zero count which normally occurs only at the beginningof a series of commands.

The count of the counter, 26 is indicated on' leads 29, 30, 31 and 32and terminal M. Each of these five connections has a logical l or a highvoltage, if the statement to which the lead or terminal is connected istrue and accordingly the counter 26 by being a five bit counter has fivebinary stages with the lead 29 being connected to the first stage, thelead 30 to the second,

the lead 31 to the' third, the lead 32 to the fourth and the terminalM'to the fifth. For counts of the counter 26 representing in binary formeven decimal numbers 0, 2, 4, 6, 8 10, etc., the lead 29 will'have alogic l thereon while for odd numbers it will be a logic 0; fornumerical numbers or counts 0, 1, 4, 5, 8, 9, 12 and 13, etc., the lead30 will be a logic l; for numbers 0, 1, 2,

3,8, 9, and 11,etc., the lead 31 will be a 1 while the lead 32 fornumerical counts of 0 through 7 and 16-23 will be a logic 1. Inaddition, the terminal M will be a 1 for all counts of the counterexcept fora count of 16 through 31. A terminal L will be a logic 1 ifthe count of the counter is not zero. The terminal'l will have a logic 1whenever the count of the counter is 16 or 4 more, it being understoodthat the counter has amaximum count of Bi and will spill over to a zerocount with the next added input pulse.

Connected to the leads 29, 30, 31 and 32 is a three deck switchgenerally indicated by the reference, numeral 33 with each deck havingten terminals. The deck 33a has its terminals interconnected as shown tothe lead 29, the deck 33b has its terminals connected as shown to theleads and 32 and the deck 33c'has its terminals also connected as shownto lead 31. The three selectorarms of the switch are connected togetherto a terminal J as indicated by the solid electrical line while theirmechanical connection is shown by the dotted line and requires that thearms be moved together to have the same position on each deck.

In addition to the AND gates 22 and 23 the backlash circuit of thepresent invention includes eight more AND or NAND gates indicated by thereference numerals 34, 35, 36, 37, 38, 39, 40, 41 and 42; three OR orNOR gates 43, 44 and 45; two exclusive OR gates 46 and 47 and aninverter 48. ,The gates and various components are interconnectedtogether either by the lines as shown or by having similarly letteredcircled termi nals connected together. As an example, the terminal A isconnected to one input of the AND gate and 7 one input of the exclusiveOR gate 47; the terminal B is connected as an input to the AND gate 36and to the invertor 48 which inverts the logic signal on terminal B andsupplies it as an input to each of the AND gates 37 inputs are logic 1and provides a logic 1 output whenever any one of its inputs is a logic0. a

In the operation of the backlash compensation circuit, with the switch24 being positioned in contact with the terminal 25 wherein no backlashcompensate is desired, the command consisting of the number of pulsesdesired and the direction are introduced into the counter 15 to set thecounter to the number and a start pulse (a logic 1) is provided on thelead 19. In addition, the up-down counter 26 is set to a zero countby-switch 28. In this situation terminals 8, F, J, K, M, 17 and lead Aall are a logic 1 with the remainder of the terminals having a logic 0.The oscillator receives the command to produce pulses on the lead 20with the pulses being directed to the gates 22 and 23. The assumedcommanded direction is in the X direction and thus the signal from theexclusive OR gate 46 to the inputs of the AND gates 22 and 23 is a logic1 and hence only the gate 22 will pass the pulses on the-l- X lead tothe motor control to effect stepping of the motor- 12 in .the

with eachpulse reducing the count of the counter 15.

forth by the input in the X direction to so move the part the commandeddistance in the commanded direction.

For non-backlash compensation movement in the X direction again thenumber of pulses desired is supplied on the input lead 16a to set thecounter count thereto while the X direction is set. Accordingly, theterminals F, J, K, M and lead A are l with the remainder being 0 so thatwhen a start pulse is supplied on the lead 19 to the gate 45, theoscillator will begin producing pulses. The exclusive OR gate 46 has a 0output (both inputs 0) which blocks pulses through the gate 22 butpermits pulses to pass through the gate 23 to the minus direction leadof the motor control. The motor will thus step in the negative directionthe number of steps desired until the count of the counter, afterreceiving at the terminal 17 the same number of pulses as the motor,becomes a zero count. The terminal F then becomes 0 stopping furtherpulses.

For the condition where it is desired to supply backlash compensation byalways moving the part 13 to its final commanded position in the samedirection, it is assumed that this direction is the X direction in thespecific embodiment herein disclosed and that the compensating movementin the -X direction-consists of 16 steps though, of course, a differentnumber of steps may be used if desired but in any event the number ofsteps utilized produces a movement which is not less than the actualbacklash. The mode switch 24 is positioned to engage terminal G.Accordingly, for a command wherein movement is in the X direction for adesired number of steps, the counter 15 is set for this number and thedirection is set to +X which makes terminals B, F, G, J, K, M and lead Aa logic 1 so that when a pulse appears (a change from logic 0 to 1 to 0)on the lead 19, the oscillator will supply to the motor control throughthe gate 22 to effect movement of the machine part 13 in the +Xdirection. The count of the counter 15 is decreased by one with eachpulse through the terminal 17 until the count of the counter reacheszero at which time terminals A, B, G, J, M are 1 which causes stoppingof the oscillator 18 by the terminal F, becoming 0.

For the operation wherein it is desired to move in the X direction withbacklash compensation being effected by the part 13 always attaining itsfinal position after a movement in the +X direction, the mode switch 24is set to engage terminal G. The counter 26 isset at zero as in thethree above-described movements if it is not already at a zero count.Upon the initiation of the start command on the lead 19, after thecommands are in the counter 15, terminals F, G, J, K, M and lead A areeach 1. The oscillator. will deliver pulses on lead 20 and these pulseswill be delivered to the counter 26 through the gate 42 by terminals Dand 0 being 1 for each pulse. The terminal N is also a 1 and thus thecounter 26 will count up one count for each pulse. Meanwhile each pulsesupplied to the counter 26 is directed to the minus axis of the motorcontrol by the exclusive OR gate 46 having both 0 inputs which producesa 0 output and enables gate 23 to pass pulses from lead 20 whileprohibiting pulses from passing through the gate 22. Further, duringthis initial backlash movement wherein there is a determined movement inthe X direction, the pulses supplied to the motor control are preventedfrom being applied to the counter 15 by the input of the gate 34connected to the terminal 0 being a l which prevents each pulse which isalso a 1 from being a l at the count terminal H7. The motor will becaused to step in a X direction for in this specific embodiment 16 stepswith each step being counted by the counter 26 but not counted by thecounter 15.

When the counter 26 achieves a sixteen count which makes the terminal Ia l, the terminals F, G, l, J, L will each be a logic 1 preventingfurther pulses to the counter 26 through gate 42 and causes the motor tostep the number of pulses in the negative direction dictated by thenumber in the counter 15 with the count of the counter decreasing by onefor each pulse as the.

gate 34 passes pulses therethrough. Upon the counter 15 reaching a countof zero, the terminals A, E, F, G, l, J, K, L, N, 0 will be 1, theoscillator 18 will continue to produce further pulses but they will beprevented from being applied to the count terminal 17 by the gate 34 asits input terminal 0 is a logic 1. In addition, the terminal E is a lwhich makes the output of the exclusive OR gate 46 a l and enables thegate 22 while disabling the gate 23. Further, the gate 42 conductspulses to the counter 26 and as the terminal N is a 1 the counter willcount up. However, as it is a five bit counter it will count up from 16to 31 where it spills over and its count becomes zero. During its countfrom 17 to 32 (or zero) the terminal L is l but when the count becomeszero the terminals I and L then both become 0. At this time, terminalsA, G, J, M are each 1 with the terminal F being a 0 and this causes theoscillator to stop producing pulses.

Accordingly, in this manner of backlash compensation whenever there ismovement in the direction (X) that is other than the direction (+X) inwhich the part is to be moved to its final position, there is initiallyadded a set number of pulses (16) in the commanded direction, then thenumber of pulses desired are supplied in the commanded direction andthen the same set number of pulses are supplied to the motor control tocause the motor to move to its final position in the other thancommanded direction. It will also be noted that the counter 26 afterthis operation has again a zero count.

For the other mode of backlash compensation, the selection switch 24 isplaced to make the terminal H a logic 1 and the switch 33 is set to themeasured amount of backlash that appears in the lost motion 14. In theexample hereinafter given, the amount of backlash has been assumed asbeing equal to three increments of movements or steps and the contactarms of the switch 33 are shown in solid lines at this three stepposition. With the switch 33 shown, any number of steps from zero tonine may be selected by the position of the switch clockwise about thecontacts shown.

In this mode of backlash'compensation operation, the compensatingmovement is provided whenever the desired movement of the machine partis reversed and is effected prior to performing the commanded movement.Thus, for a movement in the +X direction followed by another movement inthe +X direction there is no backlash compensation in the secondmovement but if the third movement is in the minus direction, backlashcompensation of three steps is inserted. Moreover, if the next movementis also in the negative direction, no compensation is provided and it isnot provided until the commanded movement in the plus direction when itis effected prior to performing such movement.

performing the commanded operation.

Accordingly, for a +X movement the selection switch 24 is set at theterminal H, the exact amount of backlash switch 33 is setat three steps,the counter 26 is set to a zero count and the input 16 provides thenumber of increments desired together with acommand for +X directionmovement.

The terminals B, F, H, J, K, M, and lead A are each lso that theoscillator begins providing pulses upon thestart signal appearing on thelead 19 and the pulses are directed through the gate 22 to the +X axisof the, motor control. They are prevented by the gate 34 from beingcounted by the counter 15. However, the

- gate 42 passes each pulse to the counter 26 which by reason of the upterminal N being 1 effects the counter to count up. The counter willcount pulses until the count of three is reached when the terminal Jbecomes to thecounter26 as terminal 0 is zero. Pulses will be supplieduntil terminalA becomes 1 and terminal Abecomes 0 at which time theoscillator will be prevented from producing further pulses and the logicstates for thefterminals at this stop position have terminals A, G, H,L'and M all being 1 with the remainder including lead A being 0.,

if the next command movement is again in the +X direction the counterand direction control are set by the input information which causesterminals-B, F. H, J, L, M and lead A' to be 1 and as the terminal 0 is0, pulses are not passed by the gate 42 into the counter.

Thus terminal .1 remains a 0 while gate34 permits pulses to pass T tothe counter 15. Accordingly, the pulses, from the oscillator will passdirectly to. the counter 15 andthe +X direction of the motor controluntilthe count of counter 15 becomes zero when it will stop theoscillator by the terminals A, B, H, L and M beingl.

It will thus be appreciated that as the counter 26 had the count equalto the setting of the switch 33 that backlash was prevented from beingintroduced prior to If, as the next movement with the counter 26 havinga count of three, a negative (-X) movement is desired, the input 16 setsthe counter 15 and through the lead 16b, the direction to -X.Accordingly, terminals C, F, H, K, L, M and O are 1 together with leadA. The oscillator 18 begins providing pulses on the lead 20 and thegate'42 permits the pulses to pass to the count terminal of the counter26 while the gate 34 blocks the pulses to the counter 15. The terminal Cby being a 1 causes the counter 26 to count down for each pulse. Alsothe gate 23 enables pulses to pass while the gate 22 inhibits passage ofpulses. This will continue until the terminal L changes ma 0 indicatinga zero count of the counter 26 at which time the terminals F, H, J, K, Mand lead A will become a 1 to cause the oscillator to continue to haveits pulses directed through the gate 23 to the X lead and also throughthe gate 34 to the count terminal 17 but not through the gate 42 to thecounter 26.

Upon the count of the counter 15 becoming zero, the terminal A and leadA change to l and 0 respectively so that the logic states of onlyterminals A, H, J and M are l "which causes the oscillator to ceaseproducing pulses.

If the next command is in the negative direction no backlash will beintroduced by reason of the count of the counter 26 being 0 so thatterminals J and L are 0 and 1 respectively. However, if the movement isto be in the +X direction then the above-described first mode ofoperation will be effected where backlash is introduced prior toaccomplishing the above-noted operation. i

It will thus be appreciated that in the third selection of operationwhenever there. is a reversal of commanded movement that the system willprovide the exact amount of backlash compensation depending upon thecondition of the counter whether its count is zero or whether it has acounty equal to the exact amount of backlash. Thus there is obviated thenecessity for providing circuitry which remembers the immediatelypreceding movement and compares the .commanded movement with thepreceding movement to determine if backlash is required. Accordingly,the utilization of the counter 26 for not only providing for the amountof backlash in two difierent modes of backlash compensation but also forproviding control over the introduction of backlash simplifies thepresent system in addition to enabling its use with presently existingnumerical controlled systems. Moreover, the system as described issusceptible to economic manufacture use in integrated circuits.

Variations and modifications may be made within the scope of the claimsand'portions of the improvements may be used without others. I

I claim:

l. A compensating system for use with a motor for incrementally moving amachine part in either of two directions wherein there is backlashbetween the motor and the part, with the backlash being predeterminableas a number of incremental movements of the motor, for overcoming thebacklash comprising means for commanding the motor to move the part anumber of equal length increments in a selected direction and means forselecting one of threemodes of backlash compensation consisting of a. nobacklash compensation, 1

b. backlash compensation for moving the part to each of its finalcommanded positions in the same direction, and

c. backlash compensation by adding the predetermined motion to thecommanded motion in the same direction as the commanded motion only whenthe commanded motion is reversed from the immediately precedingcommanded motion, in which the system includes a counting means havingmeans for providing a first signal when the counting means has a zerocount, a second signal when the counting means has a predetermined countequal to the number of movements in the backlash and a third signal whenthe'counting means has a greater count than the predetermined count andmeans for using the first and third signals for determining if backlashcompensation is to be effected when mode b is selected and settingthe'extent of the backlash compensation and for using the first andsecond signals for determining if backlash compensation is to beeffected when mode c is selected and setting the extent of the backlashcompensation.

10 of the counting means equals the selected count inhibits backlashcompensating directed movement for a commanded movement in the onedireciton.

4. The invention as defined in claim 2 in which the presence of thefirst signal indicating that the count of the counting means is zeroinhibits backlash compensating directed movement for a commandedmovement in the other direction.

1. A compensating system for use with a motor foR incrementally moving amachine part in either of two directions wherein there is backlashbetween the motor and the part, with the backlash being predeterminableas a number of incremental movements of the motor, for overcoming thebacklash comprising means for commanding the motor to move the part anumber of equal length increments in a selected direction and means forselecting one of three modes of backlash compensation consisting of a.no backlash compensation, b. backlash compensation for moving the partto each of its final commanded positions in the same direction, and c.backlash compensation by adding the predetermined motion to thecommanded motion in the same direction as the commanded motion only whenthe commanded motion is reversed from the immediately precedingcommanded motion, in which the system includes a counting means havingmeans for providing a first signal when the counting means has a zerocount, a second signal when the counting means has a predetermined countequal to the number of movements in the backlash and a third signal whenthe counting means has a greater count than the predetermined count andmeans for using the first and third signals for determining if backlashcompensation is to be effected when mode b is selected and setting theextent of the backlash compensation and for using the first and secondsignals for determining if backlash compensation is to be effected whenmode c is selected and setting the extent of the backlash compensation.2. The invention as defined in claim 1 in which the counting means is anup-down counter and in which there are means for causing said countingmeans to count up for each backlash compensating directed movement ofthe motor in one direction and to effectively count towards a zero countfor each backlash compensating directed movement of the motor in theother direction.
 3. The invention as defined in claim 2 in which thepresence of the second signal indicating that the count of the countingmeans equals the selected count inhibits backlash compensating directedmovement for a commanded movement in the one direction.
 4. The inventionas defined in claim 2 in which the presence of the first signalindicating that the count of the counting means is zero inhibitsbacklash compensating directed movement for a commanded movement in theother direction.